ABSTRACT: Following a reported mortality event of European flat oysters Ostrea edulis in southwestern England in December 2010, a sample of 30 oysters was examined using histology and molecular techniques. Histological examination of the oysters revealed the presence of microcell stages in the haemocytes and connective tissues of 3 out of the 30 animals examined. One animal showing marked haemocyte infiltration of the connective tissues was considered to be infected with Bonamia ostreae based on the presence of small uninucleate microcells measuring approximately 1 to 1.5 µm in diameter. Two other oysters were considered by histology to be infected with B. exitiosa. Infected haemocytes contained up to 5 microcells, measuring approximately 2 to 3 µm in diameter with a central or subcentral nucleus. Rarely, larger plasmodia-like multinucleated stages were noted in the haemocyte cytoplasm characterised by its irregular shape and increased eosinophilic cytoplasm. Haemocyte infiltration of the connective tissues surrounding the digestive gland and the mantle was noted along with necrosis of the tissues associated with the infection. Molecular analysis of the infected animals confirmed the presence of B. exitiosa in the sample. This study describes the parasite from flat oysters cultured in the UK; subsequent targeted sampling has not detected the parasite in flat oyster populations at this or other sites within the UK.